The high wear resistance of Cr 3 C 2 -NiCr coatings is reliant on the formation of a dense coating containing a high percentage of carbide grains, with minimal carbide degradation. Such coating characteristics are typically achieved through the use of high velocity oxygen fuel (HVOF) spraying. The propane fuelled, manually operated HIPOJET 2700 HVOF system is one of a suite of smaller sized commercial HVOF systems recommended for smaller job shops. However, few works have characterised the properties of carbide composite coatings produced with this system. In this work a full factorial design of experiment analysis was used to assess the effect of key operating parameters on the quality of Cr 3 C 2 -NiCr coatings. The combustion parameters (fuel and oxygen flows) were fixed at the manufacturers recommended settings in order to focus on the effect of nozzle length, powder feed rate and, spray distance. The effect of these variables on the porosity/oxide content, carbide content, microhardness, coating thickness, and relative deposit efficiency is discussed.

During heat treatment of Cr 3 C 2 -NiCr thermal spray coatings, regions of carbide dissolution have been observed to precipitate very small grains that grow into finely structured carbide networks. This study investigates the potential of developing tailored submicron carbide composites based on that process. The approach taken is to spray a conventional Cr 3 C 2 -NiCr powder under high-power plasma conditions to generate a supersaturated solid solution of Ni-Cr-C from which carbides could be precipitated via heat treatment. Preliminary trials assessed the effect of a broad range of plasma parameters to determine what conditions generate the greatest carbide dissolution with the lowest carbon loss. Follow-up trials investigated the most promising parameters in more detail to determine the effect of spray distance, with and without shrouding, on carbon loss and carbide dissolution. This paper presents the results from one of these trials in which an Ar-H 2 plasma was sprayed with a low-velocity nozzle. Gas shrouding had a minimal effect on carbide dissolution but was effective in reducing air entrainment in the plasma and the degree of decarburization.

Considerable effort has been made to translate the beneficial properties of bulk Ti(C,N)-based hardmetals to wear resistant thermal spray coatings. Such efforts have focused primarily on as-sprayed coatings. However, past work has shown that hardmetal coatings can undergo significant changes when operated at elevated temperature for extended periods. This work characterised the microstructural changes in a HVOF sprayed (Ti,Mo)(C,N)-Ni coating treated in air at 700°C for up to 30 days. The microstructural development of the carbonitride phase was very subtle. Image analysis indicated that the Mo-rich rim phase underwent the greatest degree of dissolution during spraying and precipitation with heat treatment. Dissolution of the carbonitride phases during spraying led to significant alloying of the Ni binder. Rapid recovery of the Ni binder composition occurred after one day of treatment, but it retained a higher steady state degree of alloying relative to the starting powder.

This work assesses the effect of heat treatment on the oxidation resistance of blended-powder CrC-NiCr coatings produced by HVOF spraying. The as-sprayed coating phases oxidized independently with NiCr rapidly forming nickel oxide, which was subsequently consumed by a solid-state reaction with Cr 2 O 3 . The NiO formed two distinct morphologies: large bulbous oxides on thin regions of exposed alloy and thin, flat oxide layers on large alloy regions. Heat treatment led to sintering of the splats and diffusion of Cr from the carbide phase into the NiCr alloy, increasing Cr to the point where Cr 2 O 3 dominated the oxide composition from the earliest exposure time. The implications of the change in oxidation mechanism and oxide morphology are discussed.

Cr 3 C 2 -NiCr thermal spray coatings have been extensively applied to mitigate erosion in high temperature applications such as aircraft and power generation turbines. Much laboratory based erosion research has been conducted under ambient temperature and mild erosion conditions. However, little has been presented about the coating response under the high temperature, high velocity erosion conditions typically experienced in industrial applications. This work presents the mechanisms of high velocity erosion based on experiments conducted under realistic service conditions. Single impact studies were carried out on a range of Cr 3 C 2 -NiCr coatings to assess the variation in erosion mechanism with phase degradation and starting powder morphology. Comparisons were made between the coating response in the as-sprayed state and after long-term heat treatment to determine how the erosion response changes as a function of exposure time in-service. Erosion of the as-sprayed coatings was heavily influenced by splat boundary related mass loss mechanisms. This was accentuated by in-flight carbide dissolution in the coatings based on agglomerated/sintered powders. Heat treatment led to splat sintering and a transition in the erosion response towards more microstructural based erosion mechanisms. The variation in erosion response as a function of microstructural development with heat treatment and starting powder type is presented.

This paper investigates the oxidation behavior of HVAF and HVOF sprayed Cr 3 C 2 -NiCr coatings under isothermal conditions. Over the range of 700 to 900 °C, all HVAF coating-only samples showed initial rapid mass gain before levelling off in a manner indicative of the formation of a protective oxide layer. The kinetics of the process were complex and did not conform to linear, parabolic, or logarithmic rate laws. The HVOF coatings exhibited a similar response. Where variations and anomalies are observed, they are attributed to differences in microstructure, composition, and spraying equipment. Paper includes a German-language abstract.

Thermal spraying has been used to protect many steel structures from aqueous corrosion using Zinc and Aluminium, and to some extent their alloy coatings to provide galvanic protection. The lifetimes of the coatings can approach 50 years even when exposed in severe marine environments. Zinc coatings work by continuously sacrificing themselves and slowly dissipating over time. Aluminium coatings passivate more readily and form a barrier layer, the passivity makes them less able to protect damaged areas and to self heal. A new ternary coating system involving Aluminium, Zinc and Magnesium has been shown to be capable of providing both a passive barrier layer as well as being able to give galvanically active protection. Salt spray tests have shown that the resistance to red rust of these new coatings increases by 300% over similar thicknesses of the separate metal coatings. Processing by arcspray is straightforward and both adhesion and deposition efficiency are better than where Zinc is sprayed alone.